In a fuel cell power system, fuel cells are generally maintained at a suitable temperature even when the power system is out of operations to prevent their electrolyte from denaturation. For example, in a phosphoric acid electrolyte fuel cell power system, a fuel cell power unit or a fuel cell stack comprising a plurality of fuel cells is generally maintained at about 100.degree. C. by a suitable heating means such as electric heaters, circulating heated gases and the like to prevent the phosphoric acid electrolyte from denaturation.
When starting up such a phosphoric acid electrolyte fuel cell power system, the fuel cell stack is firstly heated by the heating means to a temperature of about 120.degree. C. at which fuel cell reactions may take place, and then process gases, i.e., fuel gas and process air are fed to the fuel cell stack to heat the same with heat of the fuel cell reactions. At the same time, a direct current load resistance is switched into an output circuit of the power system as a dummy load. When the temperature of the fuel cell stack reaches its optimum operating temperature of about 190.degree. C., the fuel cell power system is operated in a steady state maintaining the rated output and the electric power generated is supplied to a load. The dummy load is then switched off.
During heating the fuel cell stack with the heat of reactions under load, however, the electric power generated is wastefully consumed as heat, resulting in lowering of an efficienoy of power generation.
On the other hand, when stopping the operation of the fuel cell system, the load is disconnected from the output circuit of the fuel cell stack and the load resistance is switched into the output circuit as a dummy load to prevent the fuel cells from damage resulting from an overvoltage on open circuit. This also causes lowering of an efficiency of power generation.
The above disadvantage may be overcome by use of a secondary battery as shown in FIG. 3. The secondary battery (E) is connected to the output circuit of the fuel cell power unit (FC) and a load (L) is connected to the output circuit of the power unit (FC) through an inverter (INV) and a switch (SW). However, if the battery (E) is being fully charged, it is impossible to use it as the dummy load when heating the power unit (FC) with heat of the fuel cell reactions. In addition, it is impossible with the fully charged secondary battery (E) to inhibit an overvoltage which may occur when the load (L) is disconnected from the output circuit to stop the operation of the power unit.